The valve train is the mechanical system responsible for operation of gas exchange valves in internal combustion engines. These valves are driven, either directly or indirectly, by cam lobes on a camshaft. The timing of the valve opening and closing is important to vehicle performance, as it affects torque and power output of the engine as well as emissions. Different engine speeds require different valve timing and lift for optimum performance. Generally, low engine speeds require valves to open a relatively small amount over a shorter duration, while high engine speeds and loads require valves to open a relatively larger amount over a longer duration for optimum performance. Engines without some method of variable valve timing must compromise between optimization at either low or high speed and sacrifice some performance in the non-elected range. By adding the ability to choose between different cam profiles, and thus driving the valves differently at different speeds and loads, engines are able to better optimize performance throughout a wider range of engine operating conditions.